Field of the Invention
The invention relates generally to devices and methods for assisting respiration extrathoracically and, more particularly, to extrathoracic assistance of respiration without sealing the torso area of the patient, such as premature infants, from an ambient pressure, and for assisting respiration extrathoracically in coordination with a positive airway pressure system.
Related Art
Respiratory distress can present a life-threatening condition to patients. Various systems and methods have been developed to deal with this condition including the use of constant negative pressure (CNP) ventilators, such as “iron lungs” and cuirass chambers, that compensate for a patient's loss of sufficient muscle control to force respiration. Mehta S, Hill N S. Noninvasive Ventilation. Am. J. Respir. Crit. Care Med., Feb. 1, 2001; 163(2): 540-577. CNP ventilators work on the principle of negative pressure applied externally to assist in breathing. For example, the iron lung required the patient to be encased in an airtight chamber with his/her head protruding and a seal placed between them and the chamber, whereas a cuirass provides a pressure seal around a portion of the patient's body, i.e. around the torso of the patient.
Such devices had significant drawbacks such as requirement of seals (which are not always effective), tissue damage from prolonged contact with the patient, reducing access to patients, and bulkiness. The use of such devices has been offset somewhat with the advent of positive airway pressure (PAP) through endrotracheal tubes, and continuous positive airway pressure (CPAP) devices, that may be used to improve respiratory function by decreasing the effort required for breathing. For example, during inspiration, the CPAP forces air into the lungs, and during expiration, the CPAP may assist in preventing bronchioles and alveoli from collapsing. However, the efficiency of CPAP devices alone can be limited by a number of factors including the physiological condition of the patient and the degree of assistance required. These problems can be particularly acute in patients, such as neonatal patients, with diminished lung compliance, a loss of functional residual capacity, and/or musculoskeletal limitations.
During normal breathing effort, the chest wall and abdomen both move out during inspiration and move in during expiration. This is considered a synchronous breathing pattern. If there is an inward motion of the chest wall during the inspiratory effort, with the paradoxical movement outward during expiration, it is a paradoxical or asynchronous breathing pattern.
This pattern occurs when the forces distending the lung (from diaphragmatic or respiratory muscle contraction) exceed the stability of the chest wall. As the diaphragm contracts, the negative forces pull the chest wall inward, creating an asynchronous chest and abdominal motion, and diminishing the area available for lung expansion.
Respiratory distress is a common problem for premature infants, and is related to diminished lung compliance (stiff lungs) related to the lack of surfactant and a loss of functional residual capacity (low lung volume, atelectasis). These factors increase the load on the respiratory muscles.
Additionally, developmental musculoskeletal limitations and added mechanical disadvantage due to the shape of the chest wall also predispose the premature infant to ventilatory challenge. The ribcage is more compliant in immature infants than older children or adults; thus, preterm infants are at greater risk for a paradoxical breathing pattern, particularly when they have stiff lungs, or respiratory distress syndrome (RDS). Incomplete ossification of the ribcage and underdevelopment of respiratory muscles predispose the thoracic wall to distortion since it is unable to resist the collapsing force created with inspiratory efforts. In this regard, the changes in the configuration of the chest wall with gestational age are also significant. The circumference of an infant's chest wall is more circular, and the ribs are placed more horizontally than those of the adult. This leaves the diaphragm and intercostal muscles at a mechanical disadvantage with respect to expanding thoracic volume. In addition, the chest wall of the infant is more cartilaginous, and therefore more compliant than in the adult. The relationship between high chest wall compliance and low lung compliance results in reduced thoracic volume, and thus reduced functional residual capacity (FRC). Additionally, respiratory muscle efforts can be inefficient and often ineffectual, causing distortion of the thoracic cage and retraction of the anterior chest wall rather than resulting in sufficient inspiratory volume. Together these issues result in the chest wall tending to collapse inward during inspiration as opposed to moving outward in phase with the abdomen.
In light of the above factors, a preterm infant will often breathe in a paradoxical pattern even in the face of a relatively low, or even a normal, inspiratory effort. In contrast, due to the more rigid chest wall, it would take a much larger inspiratory effort to create an inward or paradoxical motion of the chest wall during inspiration in a term infant or an adult.
Asynchronous breathing is inefficient. The loss of the stenting chest wall diminishes the tidal volume and FRC. This further increases the effort required to produce an adequate tidal volume, and the resultant increase in force generation may further increase asynchrony.
A number of surgical and ventilatory therapies have been used to support the anterior retraction of the chest wall to increase FRC and promote effective inspiration. In this regard, the “xiphoid hook,” continuous negative extrathoracic pressure (CNP) and CPAP have been shown to reduce anterior chest wall retraction and improve respiratory indices in neonatal patients with RDS.
Although somewhat effective for this purpose, complications associated with tissue fragility are of concern with the hook approach. CNP ventilation typically requires complex ventilation units and has been associated with adverse effects. Thus, CPAP delivered by way of nasal prongs (NCPAP) is currently the most common means of pressure support in spontaneously breathing neonates. While improving FRC, chest wall distortion and oxygenation, NCPAP is not completely benign and has been associated with a number of adverse effects. Complications arising from the use of nasal cannulae for respiratory support include inconsistency in, and loss of, distending pressure with an open mouth or poorly fitting nasal prongs, nasal trauma and gaseous distention of the abdomen. In the case of mechanical ventilation, positive end-expiratory pressure (PEEP) supports lung volume and the relatively flaccid chest wall. High PEEP, although effective in increasing lung volumes, thus reducing atelectrauma, may impair cardiac output, contribute to ventilation-perfusion mismatch and ventilator-induced lung injury.
Bubble-CPAP (B-CPAP) has been used for the treatment of RDS in newborn infants for a number of years. In B-CPAP the expiratory limb of the CPAP circuit vents through an underwater seal. The resulting bubbles create pressure oscillations that are transmitted back to the airway opening. The pressure delivered has a broadband frequency composition (up to 15 Hz) and amplitude on the order of 4 cm of H2O. Pillow and colleagues have shown that, compared with CPAP, B-CPAP promotes enhanced airway patency during treatment of acute postnatal respiratory disease in preterm lambs and may offer protection against lung injury. Pillow et al., Bubble Continuous Positive Airway Pressure Enhances Lung Volume And Gas Exchange In Preterm Lambs, Am J Repir Crit Care Med: 2007; 176; 63-69. They suggest that the mechanism leading to these effects may be a consequence of stochastic resonance resulting from the superposition of noise on the applied pressure signal. Other authors have suggested that the oscillatory component of the bubble waveform may augment gas exchange in a manner similar to that observed with high-frequency oscillatory ventilation (HFOV). Lee et al., A Comparison Of Underwater Bubble Continuous Positive Airway Pressure With Ventilator-Derived Continuous Positive Airway Pressure In Premature Neonates Ready For Extubation, Bioi Neonate. 1998; 73; 69-75.
It is also noted that the essential clinical criteria to remain on non-invasive respiratory support modes are effective spontaneous respiratory effort and CO2 elimination. Hypercapnia, or apnea that may be secondary to hypercapnia, are the most common reasons for progressing to more invasive forms of ventilatory support. Therefore, if CO2 retention during conventional non-invasive ventilation, such as CPAP, can be reduced or eliminated, many infants can be spared invasive mechanical ventilation and the associated potential lung injury and subsequent chronic lung and airway diseases.
In light of the above, there are still problems and disadvantages associated with the known methods of improving respiratory function, particularly in neonatal patients, including limited effectiveness of various PAP methodologies, adverse effects of prolonged treatment, and accessibility to patients undergoing CNP treatments.